In this pilot study, we begin to examine the prevalence and health effects of co-occurring chronic diseases for which there is potential therapeutic competition (i.e. the treatment of one disease worsens another disease or interferes with treatment response). We focus on therapeutic competition, one of several clinical dilemmas arising from multi-morbidity, because of the inherent potential to inflict harm. The Primary Aims are to determine: 1) the prevalence among older adults of the co- occurrence of selected chronic diseases for which there is potential therapeutic competition; and 2) whether there are differences in treatments received, and adhered to, by older adults with, vs. without, selected co-occurring conditions for which there is potential therapeutic competition. The exploratory aim is to determine whether health and disease-specific outcomes differ among older adults with a selected disease according to the treatment received for co-occurring diseases with potential therapeutic competition. The selected diseases for which we will evaluate the frequency and consequences of co-occurring diseases with potential therapeutic competition include diabetes, heart failure, COPD, and osteoporosis. For this pilot study, we focus on medications as the source of therapeutic competition. The co-occurring diseases with potential therapeutic competition with the selected diseases are chosen based on: 1) evidence; 2) biology of the diseases and treatments; and 3) disease specialty guidelines. Participants are members of 2 nationally representative longitudinal cohorts of older adults, both sampled from throughout the U.S. The Medical Expenditure Panel Study (MEPS) has over 11,000 and the Medicare Current Beneficiary Study (MCBS) over 18,000 participants age 65+ years. Both MEPS and MCBS include a wealth of longitudinal data on chronic diseases, disease treatments including medications, health outcomes, and disease-specific outcomes for the selected diseases. Results of this pilot study will heighten awareness and understanding of this important but understudied area and lay the methodological groundwork for future studies. The clinical dilemmas arising from the growing numbers of older adults with multi-morbidity mandate careful scrutiny to ensure that our diagnostic, preventive, and therapeutic armamentaria are deployed as safely and effectively as possible.